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Partial lunar eclipses explained

A partial lunar eclipse happens when the Earth moves between the Sun and the Moon, but they are not precisely aligned. Only part of the Moon's visible surface moves into the Earth's shadow.

Illustration image
Partial lunar eclipse, Los Angeles, U.S.
Partial lunar eclipse as seen from Los Angeles, California.

Types of lunar eclipses

What causes a partial lunar eclipse?

A partial lunar eclipse occurs when the Earth moves between the Sun and Moon but the 3 celestial bodies do not form a perfectly straight line in space. When that happens, a small part of the Moon's surface is covered by the darkest, central part of the Earth's shadow, called the umbra. The rest of the Moon is covered by the outer part of the Earth's shadow called the penumbra.

What happens in a total lunar eclipse?

Conditions for a partial lunar eclipse

For a partial lunar eclipse to occur, 2 celestial events must happen at the same time:

  • The Moon should be a full Moon.
  • The Sun, Earth and Moon must be aligned in almost a straight line.

Moon phases

Not every full Moon night

Partial lunar eclipses do not happen every full Moon night because of the inclination of the Moon's orbital plane. The Moon's orbital path around the Earth is inclined at an angle of 5° to the Earth's orbital plane (ecliptic) around the Sun. The points where the 2 orbital planes meet are called lunar nodes.

Eclipses can only happen near the lunar nodes. Lunar eclipses occur when a full Moon happens near a lunar node, and solar eclipses take place when a new Moon occurs near a lunar node.

Stages of a partial lunar eclipse

  • Penumbral eclipse begins: The Earth's penumbra starts covering the Moon's surface.
  • Partial eclipse begins: The Earth's umbra starts moving over the Moon.
  • Maximum eclipse: The Earth's umbra covers the outer part of the Moon.
  • Partial eclipse ends: The Earth's umbra no longer covers the outer part of the Moon.
  • Penumbral eclipse ends: The Earth no longer casts a shadow on the Moon. This marks the end of the eclipse.

What happens in a penumbral lunar eclipse?

How to see?

Partial lunar eclipses can be seen across the night-side of the Earth. No special equipment is needed to see a partial lunar eclipse. All you need is the exact date and time for the next eclipse to be seen from where you are, the weather forecast, some warm clothes and a chair to keep you comfortable while you watch the eclipse unfold.

How to view a lunar eclipse

Topics: Astronomy, Eclipses, Moon, Sun, Earth

In this Article


All about lunar eclipses

  1. Types of solar and lunar eclipses
  2. Total lunar eclipses
  3. Partial lunar eclipses
  4. Penumbral lunar eclipses
  5. How to view a lunar eclipse
  6. Blood Moon

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